Can I use a USB webcam?
Is there an official or other list of webcams that have been tested on the RPi?
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There is a list of verified peripherals (and those with issues) available here.
You don't mention the distribution you are using nor the webcam brand/model but since the RPi is so new your particular combination may not have been tried yet.
You may want to do a Google search to see if anyone has gotten your cam to work with the x86 build of Debian or Arch, as this should give you a place to start.
If you try a peripheral that's not listed, don't forget to add it at the wiki link above.